Data lake workload optimization through index modeling and recommendation

ABSTRACT

Methods, systems and computer program products are described herein that enable data workload optimization through “what-if” modeling of indexes and index recommendation. In an example aspect, a system is configured to accept a workload in the form of a plurality of data queries, extract indexable columns from the queries, generate candidate indexes from the indexable columns, select and then build the best candidate indexes. Furthermore, the system may receive a query, generate a query plan for the query configured to use one or more of the built candidate indexes, and execute the query plan to generate a query result.

BACKGROUND

The amount of raw data in all forms generated by business organizations, science researchers and the like may be quite large, on the order of hundreds of petabytes. Modern systems often gather and generate data at a rate many times greater than such data can be usefully categorized and managed. Data lakes have seen increasing adoption in such instances. A “data lake” is data storage platform configured to store such quantities of raw data in native form whether structured or unstructured. The vast scale of a data lake, along with the oftentimes unstructured nature of the data, may make it difficult to make productive use of all the information that may be gleaned from the data.

A “data warehouse”, on the other hand, typically houses structured or processed data that may be more easily manipulated for various business intelligence or research needs. A data warehouse does not, however, have nearly the same scale as a data lake, and the retrievable information may therefore be more limiting.

The overall trend, however, has been toward convergence of lakes and warehouses. For example, data warehouse offerings are increasingly offering enhanced capabilities in data diversity and scale to approach capabilities of data lakes. Data lake offerings are showing the emergence of support in data lakes for efficiently updatable and versioned relational data with change tracking, and competitive relational query capabilities at very large scale. Likewise, data lake offerings are increasingly providing support for relational tool chains for reporting, data orchestration, security, sharing, compliance, and governance.

There historically has been a huge demand for indexing support from traditional data warehouse systems to be provided on data lake systems. Though there are many ways to improve query performance in database systems, indexes are particularly efficient in providing tremendous acceleration for certain workloads because they can reduce the amount of data retrieved for a given query. However, providing indexing solutions in the context of distributed database systems and/or cloud-based architectures presents some challenges. For example, a key driver of the adoption of cloud-based models is the flexibility to store and subsequently query data using any query engine. Unfortunately, disparate query engines typically cannot use common indexes.

SUMMARY

This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

Methods, systems and computer program products are described herein that enable data workload optimization through “what-if” modeling of indexes and index recommendation. In an example aspect, a system is configured to accept a workload in the form of a plurality of data queries, extract indexable columns from the queries, generate candidate indexes from the indexable columns, select and then build the best candidate indexes. Furthermore, the system may receive a query, generate a query plan for the query configured to use one or more of the built candidate indexes, and execute the query plan to generate a query result.

In are further example aspect, indexable columns may be determined from the properties of the query. For example, indexable columns may comprise columns of the query that correspond to equality predicates, range predicates, equi-join predicates, group-by lists, projections lists, and/or other features and/or properties. Indexable columns may further be grouped according to the table corresponding to their table access from the query.

In another aspect, indexable columns are used to generate candidate indexes that may comprise filter indexes, join indexes, and/or other index types. A number of candidate indexes may be selected to be built. In one aspect, the candidate indexes may be selected based on an estimated performance increase such indexes would provide to the workload. In example aspects, the estimated performance increase may be determined based on how frequently each candidate index would benefit a query of the workload, or based on an estimate of a reduction in workload cost the candidate index would provide.

Further features and advantages, as well as the structure and operation of various examples, are described in detail below with reference to the accompanying drawings. It is noted that the ideas and techniques are not limited to the specific examples described herein. Such examples are presented herein for illustrative purposes only. Additional examples will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate embodiments of the present application and, together with the description, further serve to explain the principles of the embodiments and to enable a person skilled in the pertinent art to make and use the embodiments.

FIG. 1 depicts an example data lake system, according to an embodiment.

FIG. 2 depicts a hierarchical view of a data lake indexing and query system, according to an embodiment.

FIG. 3 depicts an example hierarchical organization diagram of index metadata on a data lake, according to an embodiment.

FIG. 4 depicts example data lake indexing and query system application programming interfaces (APIs), according to an embodiment.

FIG. 5A depicts example index metadata specification, according to an embodiment.

FIG. 5B depicts an example instance of a covering index described according to the example index metadata specification of FIG. 5A, according to an embodiment.

FIG. 6 depicts an example index state machine to support serverless stateful index operations, according to an embodiment.

FIG. 7 depicts example log operations to support concurrency control of serverless multi-user index operations, according to an embodiment.

FIG. 8 depicts an example timeline illustrating use of the example log operations of FIG. 7 to manage concurrent index manipulations by two users, according to an embodiment.

FIG. 9 depicts an example application of a Filter Index Rule to a SQL query, according to an embodiment.

FIG. 10 depicts an example application of a Join Index Rule to a SQL query, according to an embodiment.

FIG. 11 depicts an example workload and example steps for generating index recommendations for that workload, according to an embodiment.

FIG. 12A depicts an architectural overview of cost-based index tuning using a “what if” utility, according to an embodiment.

FIG. 12B depicts an example application of the “what if” utility to a SQL query, according to an embodiment.

FIG. 13 depicts a detailed schematic view of a workload optimization system, according to an embodiment.

FIG. 14 depicts a flowchart of a method for workload optimization in a distributed query processing system, according to an embodiment.

FIG. 15 depicts a flowchart of a refinement to the flowchart of FIG. 14 for generating a set of candidate indexes for each query of a workload, according to an embodiment.

FIG. 16 is a block diagram of an example computer system in which embodiments may be implemented.

The features and advantages of embodiments will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number.

DETAILED DESCRIPTION I. Introduction

The present specification and accompanying drawings disclose one or more embodiments that incorporate the features of the present invention. The scope of the present invention is not limited to the disclosed embodiments. The disclosed embodiments merely exemplify the present invention, and modified versions of the disclosed embodiments are also encompassed by the present invention. Embodiments of the present invention are defined by the claims appended hereto.

References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

In the discussion, unless otherwise stated, adjectives such as “substantially” and “about” modifying a condition or relationship characteristic of a feature or features of an embodiment of the disclosure, are understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended.

Numerous exemplary embodiments are described as follows. It is noted that any section/subsection headings provided herein are not intended to be limiting. Embodiments are described throughout this document, and any type of embodiment may be included under any section/subsection. Furthermore, embodiments disclosed in any section/subsection may be combined with any other embodiments described in the same section/subsection and/or a different section/subsection in any manner.

II. Example Embodiments

Embodiments disclosed herein bring multi-engine interoperability to data lakes, and may include guided, semi-guided, or automatic index selections to allow users to optimize their workloads. Moreover, to lower operational costs, to further enable multi-engine interoperability while also enabling multi-user concurrency, embodiments disclosed herein embody a “serverless” index management strategy. In this section, we present an overview of the operating environment of the disclosed embodiments as depicted in FIG. 1.

FIG. 1 depicts an example data lake system 100, according to an embodiment. Data lake system 100 is depicted to include generalized components dedicated to various high-level functions. Data lake system 100 includes a data ingester 112, a data digester 114, a data modeler and server 116. Data lake system 100 further includes a data lake 110 that interfaces with data ingester 112, data digester 114, and data modeler and server 116, and enables data to be written or read thereby to and from data lake 110.

Conceptually, data ingester 112 is configured to accept structured or unstructured data and store such data in data lake 110. Such data may include, for example, log files 102 (unstructured), media 104 (unstructured), files 106 (unstructured), and/or business applications 108 including any underlying schema (structured). Note, such data categories are merely exemplary. A data lake such as data lake 110 is typically configured to accept and store any type of data in its native format. Through data ingester 112, data lake 110 may come to be populated with huge amounts of data ranging into the hundreds of petabytes or even more.

Making productive use of such huge amounts of data may be enabled through the combined operations of data digester 114 and data modeler and server 116. In embodiments, data digester 114 is configured to process the unstructured data on data lake 110 to provide a structured or semi-structured and curated view of at least some of the data contained therein. Such views may thereafter be leveraged by permit data modeler and server 116 for various purposes including producing business intelligence 118, or other useful outputs. Essentially, data modeler and server 116 may be configured to operate in a manner analogous to conventional data warehouses, but over the entire data lake as processed by data digester 114.

Embodiments described herein may be implemented in various ways. For example, embodiments may be implemented in/on data modeler and server 116 to provide a data lake indexing and query system. However, other structural and operational embodiments will be apparent to persons skilled in the relevant art(s).

Further embodiments are described as follows along with motivating advantages in light of example APIs. Additionally, auxiliary data structures provided by embodiments are also described. The workload optimization embodiments disclosed herein may be implemented in conjunction with and operate using a number of different query engines. In the context of this disclosure, however, embodiments are described in terms of Apache Spark. One skilled in the art will appreciate, however, that Apache Spark is a mere example query engine and is not a necessary component of any embodiment. Other types of query engines may be present.

Embodiments of the indexing system described herein may include one or more of the following advantages:

1: Agnostic to data format. To support the most diverse scenarios, the indexing subsystem should be able to index data stored in the lake in any format, including text data (e.g., CSV, JSON, Parquet, ORC, Avro, etc.) and binary data (e.g., videos, audios, images, etc.). Moreover, the data is considered as externally managed, i.e., control over the lifecycle of the datasets is NOT assumed.

2: Low-cost index metadata management. To avoid burdening the query optimizer and the end-user, index metadata should be light-weight, fast to retrieve, and operate independent of a third-party catalog. In other words, the indexing subsystem should only depend on the data lake for its operation and should not assume the presence of any other service to operate correctly.

3: Multi-engine interoperability. The indexing subsystem should make third-party engine integration easy. To achieve this, embodiments expose (a) index state management and (b) index metadata in as transparent a way as possible.

4: Simple and guided user experience. The indexing subsystem should support diverse users including data scientists, data engineers, and data enthusiasts. Therefore, it should offer the simplest possible experience.

5: Extensible indexing. Because it is often impractical to provide all possible auxiliary data structures that aid in query acceleration, our indexing subsystem should offer mechanisms for easy pluggability of newer auxiliary data structures (related to indexing).

6: Security, Privacy, and Compliance. Because auxiliary structures such as indexes, views, and statistics copy the original dataset either partly or in full, the indexing subsystem should meet the necessary security, privacy, and compliance standards.

These advantages, particularly multi-engine interoperability, are furthered by reconsidering indexes as “derived datasets.” While indexes were traditionally built and maintained as auxiliary data structures internal to a database management system (“DBMS”), in a data lake, because there is no single “database system,” embodiments described herein treat indexes as a form of derived data—data that has been derived from one or more datasets and may be optionally used by an arbitrary query optimizer to improve the speed of data retrieval. Treating indexes as derived datasets may have very few basic assumptions: (1) that such derived datasets support basic lifecycle operations such as create, delete, (either full or incremental) rebuild, and restore, and (2) they can be leveraged for query acceleration (in particular, be readily leverage by and/or integrated into with query optimizers and execution runtimes). Therefore, embodiments support virtually any type of index including, for example, covering indexes, zone maps, materialized views, statistics, and chunk-elimination indexes. Accordingly, each of the enumerated examples herein above are properly considered when the term “index” is used herein below, but “index” may also include any other type of derived dataset that satisfies the above described assumptions. However, for the sake of context and completeness, a few examples of derived datasets are described as follows.

Covering Index. Covering indexes are efficient in scenarios where certain selection and filter columns co-occur frequently in queries. They have the following properties:

-   -   1. Non-clustered—index is separated from the data.     -   2. Covering—index contains both key columns (i.e., “indexed         columns” as the term is used herein below) and data/payload         columns (i.e., “included columns” also as the term is used         herein below); these data/payload columns are duplicated from         the original data (for “index only” query access paths).     -   3. Columnar—index is stored in some columnar format (e.g.,         Parquet) rather than a row-oriented format such as a B-tree.         This allows leveraging techniques such as vectorization along         with min-max pruning to accelerate scans over indexes.

With all columns in the query being included in the covering index either as key or non-key columns, query performance can be significantly improved. Additional physical layout properties (such as bucketization, partitioning, and sort order) can speed up workhorse operators such as filter and join that typically dominate query execution time. In embodiments, all columns marked as “indexed columns” by the user may be bucketized and (optionally) sorted.

Chunk-Elimination Index. For queries that are highly selective (e.g., searching for a single GUID amongst billions), a class of indexes called “chunk-elimination indexes” may be advantageously employed. Chunk-elimination indexes are analogous to a traditional inverted index, except that the pointer is an arbitrary URI (as opposed to a row_id) that refers to a chunk, a reasonable unit of addressable data stored in data lake (e.g., a single Parquet file or an offset range within a large CSV file). An optimizer can leverage this index to quickly prune irrelevant blocks for a query.

Materialized Views. For expensive queries with joins or aggregations, materialized views may be created as derived datasets. These materialized views can then be used transparently by the underlying query optimizer.

Statistics. In environments with cost-based query optimizers, embodiments may enable collection of statistics (e.g., histograms) a priori for columns of interest. A capable optimizer can then leverage these statistics at runtime to optimize resources.

With the benefit of the above described advantages and background, further detailed description now turns to an architectural overview of a data lake indexing and query system, according to an embodiment. More specifically, FIG. 2 depicts an example hierarchical view 200 of a data lake indexing and query system, according to an embodiment. As shown in FIG. 2, the data lake indexing and query system 200 includes a data lake 202, an indexing infrastructure 208, a query infrastructure 224 and a set of user-facing index management APIs 226. Indexing infrastructure 200 supplies and includes a log management API 214, index specifications 212, a concurrency model 216 and on top of these, a set of index creating & maintenance API 210. Query infrastructure 224 of data lake indexing and query system 200 include a set of optimizer extensions 222, an index recommendation system 220 and a “what-if” utility 218. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion regarding data lake indexing and query system 200 as depicted in FIG. 2.

In an embodiment, users can utilize indexing infrastructure 208 (available as a service or a library) to create and maintain indexes (or “derived datasets”) on their data through the index creation and maintenance API (described further herein below). For examples, users can create a non-clustered columnar covering index, specify which the columns on which to create an index, and the columns to include as data columns. Note that embodiments do not require a separate “indexing service”, because indexing infrastructure 208 can, in principle, leverage any available query engine (e.g., Spark) for index construction. As described in greater detail below, indexes and their metadata are stored on the data lake itself and for this reason, users can parallelize index scans to the extent that their query engine scales and their environment/business allows.

In embodiments, index metadata maintenance is managed by an index manager (not shown in FIG. 2) controlled through index creating and maintenance API 210. The index manager takes charge of index metadata creation, update, and deletion when corresponding modification happens to the index data, and thus governs consistency between index data and index metadata. The index manager also provides utility functions to read the index metadata from its serialized format. For example, the query optimizer can read all index metadata and then find the best index for given queries.

Embodiments may also enable primitive components underlying index creating and maintenance API 210. For example, such primitive components may comprise any or all of log management API 214, index specifications 212, or concurrency model 216.

As mentioned above and described in more detail below, support for multi-engine interoperability motivated the need to store all the indexes and their metadata on the lake. To track the lineage of the operations that take place over an index, embodiments record user operations in an operation log as described in greater detail herein below and may do so through log management API 214.

Index specifications 212 support the extensibility advantage described above, because embodiments relate to index specifications 212 that reflect the properties of the corresponding underlying indexes (or derived datasets). These are exposed via index creating & maintenance API 210 and those wishing to extend the system to include other types of indexes/derived datasets must implement support for these APIs.

Finally, concurrency model 216 exposes primitives to support multi-user and incremental maintenance scenarios using optimistic concurrency control (as described further herein below).

Discussion now turns to the other major layer above the data lake, that of query infrastructure 224. Without loss of generality, components of query infrastructure 224 are described herein as being implemented as a Scala version library as an extension of the Spark optimizer (a.k.a., Catalyst) to make it index-aware. That is, given a query along with an existing index, embodiments implemented with Spark can perform transparent query rewriting to utilize the existing index. A step on a user's side to enable optimizer extensions 222 is to execute sparkSession.enableIndexingSubsystem( ) after creating the Spark session. Because embodiments treat an index as being just another dataset on the lake, users can exploit Spark's distributed nature to automatically scale index scans. Though embodiments are described above and hereinafter in terms of Spark and Scala, it should be understood that other embodiments may employ programming languages other than Scala, and query engines other than Spark.

While embodiments described herein introduce the notion of indexing on a data lake, an important aspect of big data administration that critically influences performance is the ability to select indexes to build for a given query or a workload. To decide the right indexes for a workload, it is crucial for users to be able to perform a cost-benefit analysis of the existing indexes and any ‘hypothetical’ indexes they have in mind. Query infrastructure 224 includes, therefore a “what if” utility 218 that allows users to quantitatively analyze the impact of existing or hypothetical indexes on performance of the system. Moreover, query infrastructure 224 further includes index recommendation module 220 that exposes automated index recommendations for automating the choice of indexes in query acceleration for big data workloads. The tool takes as input a workload of SQL queries, and suggests a set of suitable indexes. Implementation details of index recommendation module 220 and the “what if” utility 218 are described in greater detail herein below.

As mentioned herein above, embodiments store all index data and metadata on the data lake itself without any external dependencies. FIG. 3 depicts an example hierarchical organization diagram 300 of index metadata on the data lake, according to an embodiment. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion regarding hierarchical organization diagram 300 as depicted in FIG. 3.

In one embodiment, all indexes may be stored at the filesystem root 302 as depicted in FIG. 3. Such an arrangement may be advantageous depending on the indexes being employed. For example, indexes may comprise materialized views which themselves may span datasets which in turn necessitates a decoupling of the dataset from the index. In another, embodiment, however, the index may be co-located with the dataset. Embodiments may implement fine-grained access control mechanisms such as, for example, copying the most restrictive access control lists (ACLs) from the datasets being indexes to achieve stricter security and compliance. Because we allow the notion of disabling public indexes, users are free to provide hints to the optimizer with “index sets,” thus allowing for A/B testing.

With continued reference to FIG. 3, each index as listed under /indexes/*/<index name> has two components:

-   -   1) the directory named_indexing_subsystem_log 308 that contains         the operational log of the index, i.e., the list of all         operations that happened on this index since its inception; and     -   2) the actual contents 322 of the index.

Notice that the contents are captured in multiple directories. This is to support functionality such as concurrent index management (e.g., snapshot isolation) and incremental maintenance (e.g., the latest index is a union of the contents of multiple directories).

FIG. 4 depicts example data lake indexing and query system application programming interfaces (APIs) 400 as exposed by embodiments in the context of Apache Spark. Note that the list is merely exemplary and should not be construed as a requirement for every engine nor for every embodiment. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion regarding APIs 400 as depicted in FIG. 4.

APIs 400 include index maintenance APIs at lines 2-8, which include APIs corresponding to actions such as create, delete, restore, vacuum, rebuild (sometimes referred to as “refresh”, particularly when the rebuild is incremental), and cancel. The deleteIndex API of line 4 corresponds to a “soft delete,” in embodiments, which tells the optimizer to not consider this index during optimization. The actual index referenced by the API calls is not permanently deleted, thus allowing the user to recover the deleted index using the restoreIndex API as shown on line 5. Alternately, the user can permanently delete an index already in a soft-delete state using the vacuumIndex API as shown in line 6. Users can cancel on-going index maintenance operations using the cancelIndex API as shown on line 8 and that may be useful if the user suspects that a maintenance job is stuck or has failed.

APIs 400 also include utility APIs for debugging and recommendation as shown at lines 11-15. These API are referred to as explain, whatIf and recommend as shown at lines 11, 12 and 14, respectively. The explain API allows users to obtain various useful information from the optimizer e.g., which part of the plan was modified, which indexes were chosen, why they were chosen, etc. The whatIf API allows users to provide the indexing subsystem with sample index configurations and get an explanation of how useful it would be if the indexes were built. The recommend API allows users to get a ranked recommendation of indexes/views that can be built for a workload of their choice.

APIs 400 also include storage and query optimizer customization configuration settings as shown at lines 18-21. These settings allow the user to override the behavior of the query optimizer and index management. For instance, by default, every index that gets created is discoverable, stored under the public folder making it accessible to all users at the workspace level. If this is not acceptable, and because indexes are accessible only to the user who created them, the user can choose private index locations and namespaces and thereafter create their private indexes and provide hints to the optimizer during optimization (e.g., by setting the configuration variables

indexing_subsystem.index.creation.[path namespace]

and/or

indexing_subsystem.index.search.disablePublicIndexes.

Having described the various APIs available in embodiments, discussion now turns to serverless index management enabled by the disclosed embodiments. As mentioned above, an advantage is a low-cost multiengine indexing subsystem that allows for concurrent index maintenance operations on an index that can be invoked by multiple engines. Although embodiments may be implemented with a server to mediate such operations, other embodiments described herein may simplify implementation by making index management “serverless” i.e., embodiments do not require a standalone server dedicated to index management tasks. The serverless functionality is achieved, in part, by storing all index information (e.g., metadata, operations on an index) in the data lake, and having that index track its own state through an index operation log incorporated into the index, and through other updates to its own metadata. Despite being serverless, embodiments enable concurrent updates through optimistic concurrency control (as described in further detail below). Further description of embodiments now turns to further description of one of these aspects: index metadata on the lake.

Interoperability is complex, as every query engine has to agree on what constitutes an index, which may require agreement between developers (and organizations/companies) working in different silo-ed ecosystems. Because the latter problem is much harder in reality, embodiments described herein prioritize a low-friction configuration for exposing index-related metadata (e.g., contents, state etc.) in a way that allows for easy integration. Exposing the state of an index or the list of operations invoked on an index through traditional means, such as a catalog service or a transaction manager service, guarantees strong consistency. However, this approach has a few major operational implications. First, it brings in service dependencies and live-site support overheads. Second, it makes integration complex because now every new engine has to depend on a third-party service. Finally, it introduces operational costs of running the service.

In consideration of these downsides, embodiments described herein trade-off metadata consistency for easier operational maintenance, i.e., the ground truth of information of an index is stored on the data lake. There are numerous ways of specifying the index information that need be stored. For example, FIG. 5A depicts an example index metadata specification 500, according to an embodiment. Index metadata specification 500 includes three parts: a contents 504, a lineage 506 and a state 508. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion index metadata specification 500 as depicted in FIG. 5A.

Contents 504 may include the type and type-specific information of the derived dataset that is useful in instantiating appropriate index interpretation logic, such as name, kind, configuration (e.g., indexed and included columns plus their types), content (e.g., physical location and layout).

Lineage 506 may include information used to track lineage of the derived dataset, e.g., HDFS data source being indexed, information needed to refresh the index with minimal information from the user, information needed to perform index/view selection, and descriptive history of an index.

State 508 may include state information pertaining to the derived dataset, e.g., global information such as Active and Disabled, and transient information such as Creating and Deleted.

FIG. 5A also includes an operation log 510. Operation log 510 is described in greater detail herein below in conjunction with a description of multi-user concurrency control.

FIG. 5B depicts an example instance of a covering index 502 described according to the example index metadata specification 500 of FIG. 5A, according to an embodiment. Those skill in the art will readily understand the great majority of the metadata of covering index 502. A first thing to note, however, all metadata is stored in easily readable JSON format, which reduces dependencies for an integration with a query engine, and that provides support for spec evolution through versioning fields. Also please note plan node 514 under source node 512. Plan node 514 further includes a rawPlan node 516 that is a property of plan node 514. RawPlan node 516 is the raw query plan information for covering index 502. For instance, in the case of Spark, rawPlan node 516 is the serialized representation of the logical plan.

Including the raw query plan information in rawPlan node 516 of covering index 502 offers a number of advantages. First, the raw query plan enables support for transparent index refreshes (e.g., by invoking the rebuild( ) API described above in relation to FIG. 4) without having the user provide the original query that was used when creating the index. Second, it allows a query engine to decide whether or not to utilize this index during optimization. For example, and recall that embodiments include multi-engine support, a query engine may examine the raw query plan for an index and discover that the index was created using an unsupported hash function and as a consequence omit that index from any optimized plans. Third, inclusion of the raw query plan is useful for debugging purposes.

Covering index 502 also includes a state node 518 which, as mentioned above, tracks the state of the index thereby enabling embodiments to be serverless. There are numerous ways of managing index state under a serverless paradigm. For example, FIG. 6 depicts an example index state machine 600 to support serverless stateful index operations, according to an embodiment. State machine 600 includes the following transient states: creating 602, restoring 604, deleting 606, refreshing 608, and optimizing 610. State machine 600 also includes the following stable states: active 612, deleted 614 and empty/DNE 616 (where ‘DNE’=‘does not exist’). Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion of index state machine 600 as depicted in FIG. 6.

Because embodiments are implemented in a serverless paradigm, there is of course no server to maintain or track index states. Accordingly, embodiments manage index states by according to the state transitions illustrated in index state machine 600 of FIG. 6. Note that states active 612, deleted 614 and empty/DNE 616 are stable states, whereas the other states are transitioning. These state transitions are described as follows:

Creating 602: Assuming no index exists, the state machine starts in state empty/DNE 616. When a user invokes the createIndex( ) API as described above in conjunction with FIG. 4, the index being created enters state creating 602 from state empty/DNE 616. If for some reason, while the index is still in state creating 602, the user cancels index creation by issuing the cancelIndex( ) API, then the index goes back to state empty/DNE 616.

Active 612: Once the index is created successful, the index transitions to state active 612 and becomes visible (the index is not visible when it is in state creating 602). An index ordinarily spends most of its time in state active 612.

Refreshing 608: An existing index may be refreshed/rebuilt via the indexRebuild( ) API described above. Although the terms refresh and rebuild are basically used interchangeably herein, the term “refresh” is often applied an incremental rebuild. Note, refreshing does not block index visibility—consumers of the index may continue to access the current active copy of the index until refreshing is finished.

Deleting 606: A user can delete an index using the deleteIndex( ) API described above. During the delete operation, the index enters state deleting 606. As described above, a delete operation is a soft delete only (for the sake of speed) and has the effect of making the index invisible/unusable.

Deleted 614: Upon completion of the deleteIndex( ) call, the index enters state deleted 614.

Restoring 604: Because a delete is only a soft delete, a restoreIndex( ) command may restore the index at which point the index enters state restoring 604 and upon completion, the index again enters state active 612. Again, the index is not visible when it is in the Restoring state.

Optimizing 610: A user can further choose to optimize the index via an optimizeIndex( ) API. For example, one optimization is index compaction, where (small) index blocks generated incrementally are merged into larger ones to improve index read efficiency.

In a multi-user scenario, clearly some index states conflict with one another (i.e., the index cannot take on certain states for different users at the same time). For example, if an index is in state deleting 606, refreshing 608, or optimizing 610 in one user session, the cannot be in state restoring at the same time in another concurrent user session. This can be appreciated because the index can only move to deleting 606, refreshing 608, or optimizing 610 from active 612, whereas it can only enter restoring 604 from deleted 606. If two API calls can lead to conflicting index states, they are incompatible. Table 1 illustrates the compatibility matrix of APIs disclosed herein, and shows the API calls of one user (e.g., when reading across) that are incompatible with that of a second user (reading downward), where C=create, D=delete, O=optimize, RF=refresh, RS=restore and V=vacuum.

TABLE 1 Compatibility matrix of index management APIs API C D O RF RS V C Y N N N N N D N Y Y Y N N O N Y Y Y N N RF N Y Y Y N N RS N N N N Y N V N N N N N Y

Although Table 1 prevents an index from reaching incompatible states in two different user sessions, it cannot prevent two different users from attempting to make conflicting changes to the index. To address this problem, embodiments ensure the index consistency through optimistic concurrency control. As described above, embodiments implement an optimistic concurrency control scheme that utilizes the example log operations 700 according to FIG. 7. The example log operations 700 include a LogOp( ) 702, a Validate( ) 704, a Begin( ) 706, a RunOp( ) 708, Commit( ) 710 and a Commit Protocol 712. Such operations are performed using the operation log 510 of FIG. 5A, and are described herein below:

-   -   LogOp( )—records the index manipulation that a user is going to         attempt to operation log 510     -   Validate( )—validates whether the index is in a suitable state         that allows for the desired manipulation (e.g., one cannot         delete an index that does not exist)     -   Begin( )—assigns an id to the index manipulation with the         corresponding transitioning state     -   RunOp( )—records to operation log 510 that the desired         manipulation is now running     -   Commit( )—records the id of the finished index manipulation to         operation log 510, including the corresponding final stable         state

Commit( ) relies on the atomicity of renaming a file in a cloud file system (such as, e.g., HDFS, Azure Storage, or Azure Data Lake) to ensure that altering index state from a transitioning state to a stable state during Commit( ) is atomic. For example, if during a commit the file corresponding to the index transitioning state is renamed, the commit and transaction as a whole may be aborted (as depicted in commit protocol 712). The transaction may be attempted again later after receiving an abort message.

An example transaction is illustrated in timeline 800 of FIG. 8 illustrating use of the example log operations of FIG. 7 to manage concurrent index manipulations by two users. Timeline 800 proceeds from left to right wherein, initially, an index is created at step 802, and data is added at step 804. At step 806, a user (User 1) tries to refresh an existing index. Near simultaneously, another user (User 2) tries to delete the same index at step 808. Assuming that the deletion is expensive (i.e., time consuming), the refresh from User 1 finishes first at step 810 successfully. When the deletion of User 2 attempts to commit, the log reflects the fact that the state of the index changed (due to the refresh of User 1), and the delete fails at step 812.

Embodiments are enabled to permit multiple writers using the above concurrency control mechanism, and multiple readers. For a reader of the index, any stable snapshot of the index data that has committed ordinarily suffices. To ensure consistency between the index and the corresponding data being indexed, embodiments may employ a signature-based mechanism whereby the latest timestamps of the data files are used to generate a signature (e.g., signature 520 as shown in FIG. 5A) for the index. As described more herein below, during query processing, it is possible that a query may touch a table as well as its indexes simultaneously. Using the signature of the data that is stored in the index metadata, the optimizer can ensure that the index is not stale. In a serverless regime, it is not generally possible to guarantee perfect external consistency. For example, it is possible that a query is accessing data that has been updated since the data was validated using signature 520 of the index, and there are no locks on the resources. Instead, external consistency has to be supported at the query engine level—i.e., external consistency can be supported only if the query engine can expose such an API.

The description set forth above provides a framework for indexes and a corresponding lifecycle API. Of course, such indexes are not useful unless they can be leverage at query time. Accordingly, the query optimizer of a query engine must be made aware of the existence and format of the indexes, and be enabled to properly process such indexes. As described in further detail herein below, embodiments lever indexes by incorporating new rules into Spark's rule-based query optimizer. In particular, Filter and Join Index Rules are defined, and may be used to optimize the query plan of a given query to use available indices. In the next section, we start by discussing the impact of indexes on query execution. We then present the details of the implementation and integration of indexing rules. Integration of indexing rules into other types of query optimizers, such as ones that follow the architectures of Starburst, Volcano, or Cascades would be similar.

As mentioned above, embodiments described herein focus on two workhouse

Spark operators in query processing, filter and join, merely to illustrate the concepts (i.e., other operators, such as aggregations and group by, may also be optimized in a similar way). More specifically, embodiments implement two query optimizer rules, FilterIndexRule and JoinIndexRule, that target accelerating filter and join operators in Spark query execution plans using indexes as described herein above. Although not described herein, such indexes may also be beneficial for other operators, such as aggregates on top of group-bys, among others. Herein below, we formally define FilterIndexRule and JoinIndexRule.

A FilterIndexRule works as follows: if a table scan has a filter f on top of it, we replace it by an index I if the following conditions meet:

-   -   The leading column in the indexed (key) columns of I is         referenced by some predicate in f;     -   All columns referenced by predicates in f are covered by I,         i.e., appear in either the indexed or included columns of I.

Consider an example, in FIG. 9, that illustrates an example application 900 by a query optimizer of a Filter Index Rule to a SQL query, according to an embodiment. Example application 900 assumes an input configuration 902 that includes tables 904, a query 906 and index 908. Indexes 908 include a single index, F₁. which is a Filter index. Index F₁ includes an indexed column R.a (i.e., R.a is a key column of index F₁ which is denoted by being underlined), and an included column, R.b. Having received query 906, a query optimizer may generate original query plan 912 in response. The indexing rule first searches the query plan for matches of the pattern

-   -   Scan->Filter.

In original query plan 912 generated from query 906, there are two such matches:

-   -   (M1) Scan(R)->Filter(R.a=5);     -   (M2) Scan(S)->Filter(S.d>200).

For each match, the indexing rule directs the query optimizer to further examine whether there is an index that meets the condition and if so, replace the table scan by the corresponding index. In our example, only the match (M1) has such an index: index F₁ that is defined to have an indexed column equal to R.a. As a result, the scan operator on top of the table R is replaced by a scan operator on top of the index F₁ instead, resulting in optimized query plan 914.

The JoinIndexRule works in a similar manner by looking for candidate indexes via pattern matching. However, unlike the FilterIndexRule, it is not possible to match a specific pattern except for merely matching individual join operators. When a matching join operator is found, it is inspected to see if it satisfies the equi-join condition i.e., a join condition that is restricted to be a conjunction of equality predicates between join columns.

After matching an eligible join operator O with join condition c, the next step is to find usable indexes for O. Given that both the left and right sub-plans of a join are linear, there are only two base tables in the plan tree under O. For each base table T, the following conditions are checked for each candidate index I on top of T:

All join columns in T that are referenced by c should be the same as the indexed columns of I;

All other columns referenced by the left or right sub-plan that accesses T are contained by the included columns of I.

More formally, let

and

be the candidate indexes found for the left and right sub-plan, respectively. Indexes may be further pairs by performing the following compatibility test:

-   -   Consider an index I_(l)∈         and an index I_(r)∈         . The pair of two indexes (I_(l),I_(r)) is compatible if the         indexed columns (K_(l1), . . . , K_(in)) of I_(l) and the         indexed columns of (K_(r1), . . . , K_(rn)) of I_(r) are         pairwise associative, i.e., each column pair (K_(ij), . . . ,         K_(rj)) appears in an equi-join predicate K_(1j)=K_(rj). Note,         I_(l) and I_(r) must have the same number of indexed columns.

The above described compatibility test may be understood by way of an example:

-   -   Consider the query:         -   SELECT A, D FROM T1, T2         -   WHERE T1.A=T2.B AND T1.C=T2.D     -   Suppose that we have two indexes I₁         (A,C);( )         over T₁ and I₂         (B,D);( )         over T₂. Then (I₁, I₂) is compatible. If instead of I₂, we have         index I′₂         (D,B);( )         , then (I₁, I′₂) is not compatible.

It is possible that more than one compatibility index pair exists. In one embodiment, the index pair that results in the least execution cost may be selected based on the following criteria:

-   -   If there exist compatible index pairs (I_(l), I_(r)) such that         I_(l) and I_(r) have the same number of buckets, then pick the         index pair with the largest number of buckets.     -   Otherwise, if no such index pair exists, then pick an arbitrary         index pair from the eligible pairs.

These criteria are used for a few reasons. First, when two indexes have the same number of buckets, there is no shuffling when performing the (sort-merge) join. That is, if the number of buckets differ, one index gets reshuffled into the number of buckets equal to the other. Second, generally speaking a greater number of buckets may lead to better parallelism in join execution (assuming no resource constraint).

Finally, JoinIndexRule replaces the scan operators on top of the tables by scan operators on top of the corresponding indexes in the best compatible index pair. For example, consider FIG. 10 which illustrates an example application 1000 of a JoinIndexRule to a SQL query, according to an embodiment. Example application 1000 includes input configuration 1002 which is based on configuration 902 of FIG. 9. In particular, input configuration 1002 includes table 904 and query 906 from FIG. 9. Input configuration 1002 also includes indexes 1008 which is based upon indexes 908 of FIG. 9, but which now includes two Join indexes J₁ and J₂.

Application of the JoinIndexRule then proceeds per the description herein above. In particular, because there are now two Join indexes J₁ and J₂, query plan 912 may inspected for eligible join operators. Here, query plan 912 includes the join operator 1010 with the join condition R.b=S.c. Next, the left and right sub-plans of the join are inspected for candidate indexes. We find index J₁ is applicable to the left sub-plan (due to the presence of column R.a in filter operation 1012) and index J₂ to the right sub-plan (due to the presence of column S.d in filter operation 1016). Clearly, (J₁, J₂) is the unique compatible candidate index pair. As a result, the scan operators on top of table R and table S may be replaced by scan operators on top J₁ and J₂, respectively, resulting in optimized query plan 1014.

As described above, the FilterIndexRule and JoinIndexRule define rules that permit a query engine to optimize a query plan to take advantage of corresponding indexes. The use of such rules must, however, be integrated into the query engine. As mentioned above, embodiments herein are described in terms of the Spark query engine (aka, Catalyst). As a rule-based query optimizer, integrating these rules is straightforward and mainly consists of incorporating the indexing rules into the rules employed by the optimizer, there are two decisions that need to be made: where to include the new rules, and in what order to apply the new rules.

Where to include the rules? Having the new rules in the wrong place may lead to unexpected consequences due to potential interactions and side effects between the rules. Embodiments described herein, however, merely replace base tables by eligible indexes which has no effect on downstream operators in the logical plan. Accordingly, the new rules may be applied after all other optimizer rules (i.e., after the query optimizer has otherwise completed the logical query plan).

What is the order of the rules? Because the FilterIndexRule and

JoinIndexRule each are applied after all other rules, the order is somewhat arbitrary and may be done in either order. However, embodiments may benefit from putting the JoinIndexRule before the FilterIndexRule because one may expect that the index for a join may lead to more improvement.

Having described the architecture of the indexing subsystem, lifecycle management of indexes, and usefully leveraging such indexes in a query, description turns herein below to the question of: given a known workload of queries, what index(es) would be most beneficial to create?

Embodiments described herein below provide an index recommendation framework that operates in two major steps:

-   -   1) Candidate Generation—Candidate indexes are generated based on         the characteristics of the query workload; and     -   2) Selection of the best indexes—The best indexes of the         candidate indexes are selected using either a rule-based         approach, or a cost-based approach.

whereby, given a workload of queries, a set of candidate indexes is created, and the best indexes of that set are selected and recommended for building (or, alternatively, automatically built).

This two-step process is further explored with reference to FIG. 11 which depicts an example workload 1100 and example steps 1102 for generating index recommendations for workload 1100. Workload 1100 includes queries 1104 which includes query Q1 and query Q2. Also depicted with workload 1100 is the data source being queried shown in the form of tables 1106. Example steps 1102 include steps 1108-1112 directed to Step 1: candidate generation, Step 2: search for the best index configuration and Step 3: output of recommendations, respectively. Step 1108 includes substeps 1a and 1b. Likewise, Step 1110 includes substeps 2a and 2b. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion example steps 1102 as depicted in FIG. 11.

The main idea of candidate generation a step 1108 of FIG. 11 is the following:

-   -   1. Each query of workload 1100 is inspected to determine         indexable columns (e.g., at step 1a); and     -   2. Candidate indexes are constructed using the a (e.g., at step         1b).

In an embodiment, candidate generation may proceed according to Algorithm 1 shown herein below:

Algorithm 1 Input:  

 = {q₁, . . ., q_(n)}: a workload of queries Output:  

 = {I₁, . . ., I_(n)}:candidate indexes. 1: Main: 2: // Stage 1-Generate all ‘indexable columns’ 3: foreach q ∈  

 do 4: | q.ϵ ← {c|c ∈ equality predicates of q}; 5: | q. 

 ← {c|c ∈ range predicates of q}; 6: | q. 

 ← {c|c ∈ equi-join predicates of q}; 7: | q. 

 ←{c|c ∈ group-by lists of q}; 8: | q. 

 ← {c|c ∈ projection lists of q}; 9: | q. 

 ← q. ϵ ∪ q. 

  ∪ q. 

 ∪ q. 

 ∪ q. 

 ; 10: //Stage 2-Generate candidate indexes; 11:  

 ← Ø; 12: foreach q ∈ 

 do 13: | (table T, T. 

 ) ← Group q. 

 by tables; 14: | foreach (table T, T. 

 ) do 15: | | //Generate an index targeting filter; 16: | | I_(filter) ← GenerateFilterIndex(T, T. 

 ); 17: | |  

 ← 

 ∪ {I_(filter)}; 18: | | //Generate index targeting joins; 19: | | I_(join) GenerateJoinIndex (T, T. 

 ); 20: | |  

 ← 

 ∪ {I_(join)}; 21: return  

 ; 22: 23: GenerateFilterIndex(T, T. 

 ); 24: T. 

 ← T.ϵ + T. 

 ; 25: I.indexedCols ← T. 

 ; 26: I.includedCols ← T. 

 − T. 

 ; 27: return I; 28: 29: GenerateJoinIndex(T, T. 

 ); 30: I.indexedCols ← T. 

 ; 31: I.includedCols ← T. 

 − T. 

 ; 32: return I;

Algorithm 1 is described with continued reference to candidate generation step 1108 of FIG. 11. Embodiments may be configured to determine different types of indexable columns as known in the art. In one particular embodiment, Algorithm 1 extracts the following indexable columns from a query q:

-   -   ε—columns appearing in equality predicates (i.e., x=a where x is         a column and a is a constant) of q (line 4 of Algorithm 1);     -   —columns appearing in equality predicates (i.e., x≤a or x≥a         where x is a column and a is a constant) of q (line 5 of         Algorithm 1);     -   —columns appearing in equi-join predicates (i.e., R.x=S.y where         R and S are tables) of q (line 6 of Algorithm 1);     -   —columns appearing in group-by lists (i.e., GROUP BY x₁, . . . ,         x_(m)) of q (line 7 of Algorithm 1);     -   —columns appearing in projection lists (i.e., SELECT x₁, . . . ,         x_(m)) of q (line 8 of Algorithm 1)

The set of indexable columns

of q is simply the union of the above sets of columns (and as reflected by line 9 of Algorithm 1).

Having enumerated the indexable columns at lines 3-9 of Algorithm 1, candidate indexes for each query q, of the workload W are generated from the indexable columns at lines 13-21 of Algorithm 1, a description of which follows herein below.

To construct candidate indexes from the indexable columns, the indexable columns are grouped together by their corresponding tables (accessed by the query) on line 13. Algorithm 1 then loops over each such group of indexable columns that correspond to a particular table and generates candidate indexes for each at lines 15-20.

In particular, one or more indexes are created for indexable columns corresponding to filters (line 16 of Algorithm 1) and indexes for indexable columns corresponding to joins (line 19 of Algorithm 1) if any.

The candidate indexes corresponding to filters are denoted as I_(filter) at line 16, wherein each candidate index therein includes indexed columns and included columns. The indexed columns are formed by the concatenation of the equality filtering columns in ε with the range filtering columns in

, whereas the remaining indexable columns form its included columns (as shown in the helper function GenerateFilterIndex( ) at lines 24 to 26 of Algorithm 1). The candidate indexes corresponding to joins are denoted as I_(join) at line 19, and like the filter indexes, each candidate index includes indexed columns and included columns. The equi-join columns in

form the indexed columns, whereas the remaining indexable columns form its included columns (as shown in the helper function GenerateJoinIndex( ) at lines 30 to 31 of Algorithm 1).

Upon completion of Algorithm 1, step 1108 of FIG. 11 is likewise complete, and embodiments next must determine which index configuration is best. To select the best indexes from the set of index candidates

returned by Algorithm 1, one approach is to enumerate all subsets of

and find the subset that leads to the best improvement over the workload

in terms of query execution time. This approach, however, may not be feasible in practice when is

large. Accordingly, embodiments disclosed herein apply heuristic approaches.

The first such heuristic approach is a rule-based approach whereby deterministic statistics for the candidate indexes are compared. In particular, embodiments may implement a frequency-based approach as shown in Algorithm 2 herein below:

Algorithm 2 Input:  

 = {q₁, . . ., q_(n)}: a workload of queries  

 = {I₁, . . ., I_(n)}: candidate indexes from Algorithm 1 K: the number of indexes to bereturned Output: The best K indexes to be returned from 

 . 1: // Step 1: Count; 2: foreach I ∈  

 do 3: | foreach q ∈  

 do 4: | | if I is a candidate index from q then 5: | | | I.freq ← I.freq + 1; 6: //Step 2: Merge 7:  

_(best) ← Ø; 8: Group index in  

 by their indexed columns; 9: foreach (indexed column K, index group  

_(K)) do 10: | //Merge the indexes into one single index I_(merged); 11: | I_(merged).indexedCols ← K; 12: | I_(merged).includedCols ←

 K; 13: | I_(merged).freq ← 

 I. freq; 14: //Step 3: Rank 15: Sort 

_(best) by decreasing order of I.freq for I ∈  

 ; 16: return The top K indexes in the ordered  

_(best)

Algorithm 2 may be summarized as three steps as follows:

-   -   1. Count index frequency (lines 2-5), where we count the number         of appearances of each candidate index in the workload of         queries;     -   2. Merge candidate indexes (lines 7 to 13), where we merge         indexes with the same indexed column into one single index by         combining their included columns; and     -   3. Rank merged indexes (line 15), where we sort the merged         indexes by the decreasing order of their frequencies.

Algorithm 2 then returns the top K indexes from the ordered candidates (at line 16), where K is a pre-determined given by the user.

Though often the frequency-based often provides good index recommendations, the approach may not be ideal for all workloads. First, just because a candidate index would be used frequently by the queries of a workload does not necessarily mean that the index provides a large reduction in query execution time. For example, such an index might be over a frequently accessed and small reference (i.e., dimension) table having a negligible access time. Second, while merging candidate indexes with the same indexed columns has the advantage of reducing index storage and maintenance overhead, it may be difficult to measure the efficacy without proper understanding of the overhead. To address these and other issues, embodiments may instead employ a cost-based approach that relies on modeling the query execution cost.

One embodiment of a cost-based approach to index selection relies on several basic building blocks:

-   -   1. A cost model that estimates the execution cost of a given         query plan;     -   2. A “what if” utility that returns the hypothetical query plan         and its cost without actually building the indexes; and     -   3. An efficient search algorithm that looks for the indexes that         result in minimum workload execution cost.

FIG. 12A depicts an architectural schematic 1200 of cost-based index tuning using a “what if” utility 1210, according to an embodiment. Schematic 1200 includes an index tuning module 1204 and a query optimizer 1212 that includes a “what-if” utility 1210. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion regarding schematic 1200 as depicted in FIG. 12A.

At a high level, the cost-based approach embodied by schematic 1200 of FIG. 12A works as follows. Index tuning module 1204 provides the ordered pair 1206 comprising a query q and a hypothetical index configuration C to “what-if” utility 1210. The hypothetical index configuration may include a set of candidate indexes as described herein above. “What-if” utility 1210 may, in conjunction with query optimizer 1212, generate the ordered pair 1208 that comprises a query plan P, and an estimated cost for executing plan P (denoted as ‘cost-est(P)’ in FIG. 12A). The query plan P is the plan generated by the query optimizer assuming that the indexes of configuration C have in fact been built. Likewise, the cost estimate cost-est(P) reflects an estimate of the execution cost of plan P. More detailed description of this high-level overview is included herein below in the context of FIG. 12B which depicts an example 1202 application of “what if” utility 1210 to a SQL query, according to an embodiment. Example 1202 includes tables 1214, a SQL query 1216, filter index F₁ 1218, plan P 1220, plan P′ 1222, cost R 1224 and cost R′ 1226. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion “what-if” utility 1210 as depicted in FIG. 12.

As shown in FIG. 12B the data source has two tables 1214 to be queried: R(a, b) and S(c, d), and query 1216 is a simple filter-join query applied to them. Suppose that the user wants to understand the performance impact on query 1216 by building index F₁ 1218. In an embodiment, the user may invoke “what if” utility 1210 without building index F₁ 1218. Such an invocation generally proceeds as follows:

1. The plan P 1220 returned by query optimizer is received, and filter predicates are searched for that can be evaluated using index F₁ 1218.

2. Index F₁ 1218 is determined as beneficial for accelerating the processing of the filter R.a=5.

3. The table scan on R is replaced by accessing the “hypothetical” filter F₁ 1218, and a new plan P′ 1222 is generated. Plan P′ 1222 is not executable (because filter F₁ 1218 has yet been built).

4. The cost estimation procedure is invoked on new plan P′ 1222 and its estimated cost is generated. (Again, this cost is imaginary and makes sense if F₁ were is built).

To determine the cost P, a cost model may be applied to plan P 1220. For example, suppose the cost model is configured to estimate the output size of each operator. Suppose that the size of table R is 2 GB meaning that a table scan over R incurs the cost retrieving and processing 2 GB (denoted as cost R 1224). The cost model employed by “what-if” utility 1210 may determine, on the other hand, that when the table scan of R is replaced by index F₁ 1218, then the size can be reduced to 0.8 GB which is cost R′ 1226 as shown in FIG. 12B.

In this fashion, a cost model may estimate the output size of all the operators in plan P 1220 and likewise to the operators in plan P′ 1222, sum the respective costs and determine an estimated cost for executing each plan. Having done so, one may now compare the cost of plan P 1220 and plan P′ 1222, and compute the improvement. For the example here, assume cost(P)=size(P)=2.5 GB, and cost(P)=size(P)=1.5 GB, which are the sum of the output sizes of all operators in plan P 1220 and plan P′ 1222 respectively. As a result, the improvement would be

$\frac{{{cost}(P)} - {{cost}\left( P^{\prime} \right)}}{{cost}(P)} = {40\%}$

if index F₁ 1218 were built. Algorithm 3 shown herein below illustrates an embodiment of “what if” utility 1210:

Algorithm 3: “what if” utility Input: q: a query  

 : a set of hypothetical indexes Output: cost(q): estimated cost if  

 was built. 1: Main: 2: P ← PlanByOptimizer(q); 3: P′ ← ReplaceScansByHypotheticalIndexes(P,  

 ); 4: cost(P′) ← EstimateCost(P′); 5: return cost(P′); 6: 7: ReplaceScansByHypotheticalIndexes(P,  

 ): 8: foreach hypotheticalindex I ∈  

 do 9: | if I can be matched by FilterIndexRule or JoinIndexRule over P then 10: | | Replace the corresponding scan in P by I; 11: Return the plan P′ after replacement;

There are various ways by which a cost model may estimate the query cost of a plan. In a size-based cost model as outlined above, one may rely on metadata stored in file system to obtain sizes for base table/index files. In the example described above in conjunction with FIG. 12, the sizes of both R and index F₁ 1218 may be obtained in this way. Estimating the output size of other operators may generally be accomplished by estimating the cardinality and/or selectivity of the operators (e.g., estimating the fraction of rows in a table that meet the conditions of a predicate filter). Selectivity of operators may be determined gather statistics regarding the workload and use compact data structures (e.g., histograms, sketches, random samples) to summarize the statistics. However, gathering statistics over large data is time-consuming. Moreover, it may be difficult to maintain such statistics to prevent them from becoming stale due to data updates.

Alternatively, heuristics may be employed whereby selectivity values are assigned to operators. For example, for operators whose output sizes are the same of their input sizes, such as a sort, their selectivity is simply 1.0; for the other operators such as filter or join, their selectivity may be set as 0.1.

It should be noted that the exact value of the cost estimate generated by a cost model is not terribly important for the purposes of “what-if” utility 1210. That is, having a true and accurate cost estimate is not as important as the comparability of any two estimates. It suffices if a cost model can accurately determine which of two query plans has the higher cost.

Having provided a high-level description of “what-if” utility 1210 in the context of architectural schematic 1200 of FIG. 12A, description now turns to a detailed description of an embodiment of a cost-based approach to index selection using “what-if” utility 1210. It should be noted, however, that “what if” utility 1210 has its own benefits beyond its role in index recommendation. Using “what if” utility 1210, users are able to assess the potential improvement of building a hypothetical index quantitatively, e.g., in terms of percentage. Such benefits, however, do not come for free—they lead to more accurate cost modeling and therefore increased overhead when collecting statistics.

Algorithm 4 implements one embodiment of the cost-based approach to index selection utilizing, for example, “what if” utility 1210 as embodied by Algorithm 3, and as shown herein immediately below:

Algorithm 4 Input:  

  = {q₁, ..., q_(n)}: a workload of queries;  

 = {I₁, ..., I_(n)}: candidate indexes from Algorithm 1; K: the number of indexes to be returned. Output: The best K indexes from  

 . 1: bestIndexes ← Ø; 2: minCost ← Σ_(i=1) ^(n) ptimizerCost(q_(i)); 3: foreach subset 

 ⊆  

 up to size K do 4: | cost( 

 , 

 ) ← Σ^(n) _(i=1) whatIf(q_(n),  

 ) by Algorithm 3; 5: | if cost( 

 , 

 ) < min Cost then 6: | | bestIndexes ← 

 ; 7: | | minCost ← cost( 

, 

 ); 8: return bestIndexes;

Algorithm 4 accepts as input a workload of queries

, a set of candidate indexes

as enumerated by Algorithm 1, and the number of indexes K to return. Each subset up to size K is enumerated (at line 3), and for each such subset

, “what if” utility 1210 is invoked to obtain estimated cost of each q_(i)∈

as if the hypothetical indexes in

were built (line 4). If the sum of the estimated cost cost(

,

) for the workload

is lower than the lowest cost currently recorded, we mark

as the best indexes and update the lowest cost so far (lines 5 to 7). Finally, the overall best subset found with the minimum estimated cost is returned (line 8). Note, summing the costs returned by the invocations of whatIf(q_(n),

) as shown at line 4 is just one example of combining query costs to compute the workload cost. In an alternative embodiment, for example, one may further assign a weight (e.g., with respect to the frequency) to each query and thereafter compute a “weighted sum” when combining the query costs.

Embodiments of a workload optimization system may be implemented in various ways to use the information derived from the queries of a workload to generate index recommendations that would benefit the workload, and to build and use such indexes for servicing the queries. For example, FIG. 13 depicts a detailed schematic view 1300 of a workload optimization system 1302, according to an embodiment. As shown in FIG. 13, workload optimization system 1302 includes a candidate index generator 1304, an index selector 1308 and a query processor 1312. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion regarding workload optimization system 1302 as depicted in FIG. 13.

As an initial matter, and as described above, workload optimization system 1302 as shown in FIG. 13 is configured to receive workload 1104 which comprises a plurality of queries, and to pass workload 1104 to candidate index generator 1304. Candidate index generator 1304 is configured to extract sets of indexable columns from the queries of workload 1104 in numerous ways. In an embodiment, for example, indexable columns may be extracted from the queries of workload 1104 per Algorithm 1 as described herein above. Candidate index generator 1304 is further configured to thereafter generate a set of candidate indexes 1306 based on the sets of indexable columns. For example, candidate indexes 1306 may be generated from the sets of indexable columns per Algorithm 1.

The generated candidate indexes 1306 are thereafter passed to index selector 1308. Index selector 1308 is configured to select which index or indexes of candidate indexes 1306 would provide the greatest performance benefit when executing the queries of workload 1104, and to provide such selected indexes to query processor 1312. Index selector 1308 may select the best indexes in a number of ways. For example, index selector 1308 may employ a frequency-based approach as set forth in Algorithm 2, and as described above. Alternatively, index selector 1308 may employ a cost-based approach utilizing the “what-if” utility 1210 of FIG. 12 to determine which indexes have the lowest cost. For example, index selector 1308 may employ an embodiment of “what-if” utility 1210 that implements Algorithm 3 as described above. Further, index selector 1308 may thereafter employ Algorithm 4 to select the best indexes using “what-if” utility 1210, and the manner described above herein. Index selector 1308 thereafter passes selected indexes 1310 to query processor 1312.

Query processor 1312 is configured to accept selected indexes 1310, to build the indexes included in selected indexes 1310 to provide built indexes 1314, to receive a query 1316, to generate a query plan optimized to use one or more of built indexes 1314, and to execute the query plan to produce a query result. For example, built indexes 1314 may be built from selected indexes 1310 by building a table including one or more key columns (i.e., the “indexed columns” as described in detail herein above in the description of Algorithm 1), and one or more data columns corresponding to the “included columns.”

After receiving query 1316, query processor 1312 is configured to generate a query plan for the query wherein, wherever possible, the query plan is modified to reference one or more of built indexes 1314 by using the FilterIndexRule and JoinIndexRule, and in the manner described above in conjunction with FIGS. 9 and 10. Query processor 1312 may thereafter execute the modified query plan to produce the final query result.

Further operational aspects of workload optimization system 1302 of FIG. 13 is described in conjunction with FIG. 14 which depicts a flowchart 1400 of a method for workload optimization in a distributed query processing system, according to an embodiment. In an embodiment, flowchart 1400 may be performed by workload optimization system 1302 of FIG. 13. Although described with reference to workload optimization system 1302 as shown in FIG. 13, the method of FIG. 14 is not limited to that implementation. Other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion regarding flowchart 1400 of FIG. 14.

Flowchart 1400 is an example method for workload optimization in a distributed query processing system, according to an embodiment. Note that flowchart 1400 may be triggered to optimize a distributed query processing system workload in various ways. For example, optimization may be triggered in response to an express request from a system administrator or automatically (e.g., based on changes to the average system workload over time, or substantial changes to the underlying data).

Flowchart 1400 begins at step 1402. At step 1402, a workload comprising a plurality of queries is received. For example, and with reference to workload optimization system 1302 of FIG. 13, candidate index generator 1304 may be configured to receive workload 1104. Flowchart 1400 of FIG. 14 continues at step 1404.

In step 1404, sets of indexable columns are extracted from the plurality of queries, wherein each set corresponds to a respective one of the plurality of queries. For example, and with continued reference to workload optimization system 1302 of FIG. 13, candidate index generator 1304 may be configured to generate sets of indexable columns, one set per query in the manner described in detail above regarding workload optimization system 1302 of FIG. 13, the description of FIG. 11, and Algorithm 1 and its detailed description, in an embodiment.

In step 1406, a set of candidate indexes are generated based on the sets of indexable columns. For example, For example, and with continued reference to workload optimization system 1302 of FIG. 13, candidate index generator 1304 may be configured to generate candidate indexes based on the sets of indexable columns in the manner described in detail above regarding workload optimization system 1302 of FIG. 13, the detailed description of FIG. 11, and Algorithm 1 and its detailed description, in an embodiment. Flowchart 1400 continues at step 1408.

At step 1408, a predetermined number of candidate indexes are selected from the set of candidate indexes based on a determination of an estimated performance increase provided by the set of candidate indexes to performance of the workload. For example, and with continued reference to workload optimization system 1302 of FIG. 13, index selector 1308 may be configured to select a predetermined number of candidate indexes from the set of candidate indexes based on an estimated performance increase the selected set provides to the performance of the workload. More specifically, index selector 1308 may operate in the manner described in detail above regarding workload optimization system 1302 of FIG. 13, Algorithm 2, or Algorithm 3 in conjunction with Algorithm 4, and their respective detailed descriptions. Flowchart 1400 continues at step 1410.

At step 1410, the selected candidate indexes are built to provide built indexes. For example, and with continued reference to workload optimization system 1302 of FIG. 13, query processor 1312 may be configured to accept selected indexes 1310 from index selector 1308 and to build such indexes to provide built indexes 1314 in the manner described above regarding workload optimization system 1302 of FIG. 13, and/or as known in the art. Flowchart 1400 continues at step 1412.

At step 1412, a query is received. For example, and with continued reference to workload optimization system 1302 of FIG. 13, query processor 1312 may receive query 1316 which may comprise a query of workload 1104 or any other query. Flowchart 1400 continues at step 1414.

In step 1414, a query plan for the query is generated, the query plan configured to use at least one of the built indexes. For example, and with continued reference to workload optimization system 1302 of FIG. 13, query processor 1312 may be configured to generate an intermediate query plan that is not configured to use any of the built indexes, and thereafter apply a FilterIndexRule and/or JoinIndexRule to the intermediate query plan to provide a query plan that is configured to use at least one of the built indexes as described in detail above regarding workload optimization system 1302 of FIG. 13, FilterIndexRule and/or JoinIndexRule particularly as described in conjunction with examples 900 and 1000 of FIGS. 9 and 10, respectively.

Flowchart 1400 of FIG. 14 concludes at step 1416. In step 1416, the query plan is executed to generate a final query result. For example, and with continued reference to workload optimization system 1302 of FIG. 13, query processor 1312 may be configured to execute the query plan generated in step 1414 to generate query result 1318. As known in the art, because the query plan was configured to use at least one of built indexes 1314, generating query result 1318 requires fewer system resources (e.g., storage space, CPU cycles and/or memory) than executing the above described intermediate query plan that is not configured to use built indexes 1314.

In the foregoing discussion of steps 1402-1416 of flowchart 1400, it should be understood that at times, such steps may be performed in a different order or even contemporaneously with other steps. For example, once at least one index is built at step 1410, embodiments may perform steps 1412-1416 while the system continues to build other indexes of the predetermined number of candidate indexes. Other operational embodiments will be apparent to persons skilled in the relevant art(s). Note also that the foregoing general description of the operation of workload optimization system 1302 of FIG. 13 is provided for illustration only, and embodiments of workload optimization system 1302 may comprise different hardware and/or software, and may operate in manners different than described above. Indeed, steps of flowchart 1400 may be performed in various ways.

For example, FIG. 15 depicts a flowchart 1500 of a refinement to flowchart 1400 of FIG. 14 for generating a set of candidate indexes for each query of a workload, according to an embodiment. Accordingly, flowchart 1500 of FIG. 15 will also be described with continued reference to workload optimization system 1302 of FIG. 13. However, other structural and operational embodiments will be apparent to persons skilled in the relevant art(s) based on the following discussion regarding flowchart 1500.

Flowchart 1500 begins at step 1502. At step 1502, groups of indexable columns are created for the respective query based on the set of indexable columns corresponding to the respective query, each group of indexable columns corresponding to a table accessed by the respective query. For example, and with continued reference to workload optimization system 1302 of FIG. 13, candidate index generator 1304 may be configured to generate sets of indexable columns, in the manner described in detail above regarding workload optimization system 1302 of FIG. 13, the description of FIG. 11, and Algorithm 1 and its detailed description, in an embodiment. Flowchart 1500 of FIG. 15 continues at step 1504.

In step 1504, a set of filter indexes and a set of join indexes are generated for each group of indexable columns. For example, and with reference to workload optimization system 1302 of FIG. 13, candidate index generator 1304 may be configured to generate a set of filter indexes and a set of join indexes in the manner described in detail above regarding, for example, workload optimization system 1302 of FIG. 13, the description of FIG. 11, and Algorithm 1 and its detailed description, in an embodiment.

Flowchart 1500 of FIG. 15 concludes at step 1506. In step 106, the set of candidate indexes comprises a union of the set of filter indexes and the set of join indexes generated for each group and for each query. For example, and with reference to workload optimization system 1302 of FIG. 13, candidate index generator 1304 may generate the set of candidate indexes by forming the union of the set of filter indexes and the set of join indexes in the manner described in detail above regarding, for example, workload optimization system 1302 of FIG. 13, the description of FIG. 11, and Algorithm 1 and its detailed description, in an embodiment.

Other operational embodiments will be apparent to persons skilled in the relevant art(s). Note also that the foregoing general description of the operation of workload optimization system 1302 is provided for illustration only, and embodiments of workload optimization system 1302 may comprise different hardware and/or software, and may operate in manners different than described above.

III. Example Computer System Implementation

Each of data ingester 112, data digester 114, data modeler and server 116, query optimizer 1212, candidate index generator 1304, index selector 1308 and/or query processor 1312, and flowcharts 1400 and/or 1500 may be implemented in hardware, or hardware combined with software and/or firmware. For example, data ingester 112, data digester 114, data modeler and server 116, query optimizer 1212, candidate index generator 1304, index selector 1308 and/or query processor 1312, and flowcharts 1400 and/or 1500 may be implemented as computer program code/instructions configured to be executed in one or more processors and stored in a computer readable storage medium. Alternatively, data ingester 112, data digester 114, data modeler and server 116, query optimizer 1212, candidate index generator 1304, index selector 1308 and/or query processor 1312, and flowcharts 1400 and/or 1500 may be implemented as hardware logic/electrical circuitry.

For instance, in an embodiment, one or more, in any combination, of data ingester 112, data digester 114, data modeler and server 116, query optimizer 1212, candidate index generator 1304, index selector 1308 and/or query processor 1312, and flowcharts 1400 and/or 1500 may be implemented together in a SoC. The SoC may include an integrated circuit chip that includes one or more of a processor (e.g., a central processing unit (CPU), microcontroller, microprocessor, digital signal processor (DSP), etc.), memory, one or more communication interfaces, and/or further circuits, and may optionally execute received program code and/or include embedded firmware to perform functions.

FIG. 16 depicts an exemplary implementation of a computing device 1600 in which embodiments may be implemented. For example, of data ingester 112, data digester 114, data modeler and server 116, query optimizer 1212, candidate index generator 1304, index selector 1308 and/or query processor 1312, and flowcharts 1400 and/or 1500 may be implemented in one or more computing devices similar to computing device 1600 in stationary or mobile computer embodiments, including one or more features of computing device 1600 and/or alternative features. The description of computing device 1600 provided herein is provided for purposes of illustration and is not intended to be limiting. Embodiments may be implemented in further types of computer systems, as would be known to persons skilled in the relevant art(s).

As shown in FIG. 16, computing device 1600 includes one or more processors, referred to as processor circuit 1602, a system memory 1604, and a bus 1606 that couples various system components including system memory 1604 to processor circuit 1602. Processor circuit 1602 is an electrical and/or optical circuit implemented in one or more physical hardware electrical circuit device elements and/or integrated circuit devices (semiconductor material chips or dies) as a central processing unit (CPU), a microcontroller, a microprocessor, and/or other physical hardware processor circuit. Processor circuit 1602 may execute program code stored in a computer readable medium, such as program code of operating system 1630, application programs 1632, other programs 1634, etc. Bus 1606 represents one or more of any of several types of bus structures, including a memory bus or memory controller, a peripheral bus, an accelerated graphics port, and a processor or local bus using any of a variety of bus architectures. System memory 1604 includes read only memory (ROM) 1608 and random access memory (RAM) 1610. A basic input/output system 1612 (BIOS) is stored in ROM 1608.

Computing device 1600 also has one or more of the following drives: a hard disk drive 1614 for reading from and writing to a hard disk, a magnetic disk drive 1616 for reading from or writing to a removable magnetic disk 1618, and an optical disk drive 1620 for reading from or writing to a removable optical disk 1622 such as a CD ROM, DVD ROM, or other optical media. Hard disk drive 1614, magnetic disk drive 1616, and optical disk drive 1620 are connected to bus 1606 by a hard disk drive interface 1624, a magnetic disk drive interface 1626, and an optical drive interface 1628, respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer-readable instructions, data structures, program modules and other data for the computer. Although a hard disk, a removable magnetic disk and a removable optical disk are described, other types of hardware-based computer-readable storage media can be used to store data, such as flash memory cards, digital video disks, RAMs, ROMs, and other hardware storage media.

A number of program modules may be stored on the hard disk, magnetic disk, optical disk, ROM, or RAM. These programs include operating system 1630, one or more application programs 1632, other programs 1634, and program data 1636. Application programs 1632 or other programs 1634 may include, for example, computer program logic (e.g., computer program code or instructions) for implementing of data ingester 112, data digester 114, data modeler and server 116, query optimizer 1212, candidate index generator 1304, index selector 1308 and/or query processor 1312, and flowcharts 1400 and/or 1500 (including any suitable step of flowcharts 1400 and/or 1500), and/or further embodiments described herein.

A user may enter commands and information into the computing device 1600 through input devices such as keyboard 1638 and pointing device 1640. Other input devices (not shown) may include a microphone, joystick, game pad, satellite dish, scanner, a touch screen and/or touch pad, a voice recognition system to receive voice input, a gesture recognition system to receive gesture input, or the like. These and other input devices are often connected to processor circuit 1602 through a serial port interface 1642 that is coupled to bus 1606, but may be connected by other interfaces, such as a parallel port, game port, or a universal serial bus (USB).

A display screen 1644 is also connected to bus 1606 via an interface, such as a video adapter 1646. Display screen 1644 may be external to, or incorporated in computing device 1600. Display screen 1644 may display information, as well as being a user interface for receiving user commands and/or other information (e.g., by touch, finger gestures, virtual keyboard, etc.). In addition to display screen 1644, computing device 1600 may include other peripheral output devices (not shown) such as speakers and printers.

Computing device 1600 is connected to a network 1648 (e.g., the Internet) through an adaptor or network interface 1650, a modem 1652, or other means for establishing communications over the network. Modem 1652, which may be internal or external, may be connected to bus 1606 via serial port interface 1642, as shown in FIG. 16, or may be connected to bus 1606 using another interface type, including a parallel interface.

As used herein, the terms “computer program medium,” “computer-readable medium,” and “computer-readable storage medium” are used to refer to physical hardware media such as the hard disk associated with hard disk drive 1614, removable magnetic disk 1618, removable optical disk 1622, other physical hardware media such as RAMs, ROMs, flash memory cards, digital video disks, zip disks, MEMs, nanotechnology-based storage devices, and further types of physical/tangible hardware storage media. Such computer-readable storage media are distinguished from and non-overlapping with communication media (do not include communication media). Communication media embodies computer-readable instructions, data structures, program modules or other data in a modulated data signal such as a carrier wave. The term “modulated data signal” means a signal that has one or more of its characteristics set or changed in such a manner as to encode information in the signal. By way of example, and not limitation, communication media includes wireless media such as acoustic, RF, infrared and other wireless media, as well as wired media. Embodiments are also directed to such communication media that are separate and non-overlapping with embodiments directed to computer-readable storage media.

As noted above, computer programs and modules (including application programs 1632 and other programs 1634) may be stored on the hard disk, magnetic disk, optical disk, ROM, RAM, or other hardware storage medium. Such computer programs may also be received via network interface 1650, serial port interface 1642, or any other interface type. Such computer programs, when executed or loaded by an application, enable computing device 1600 to implement features of embodiments described herein. Accordingly, such computer programs represent controllers of the computing device 1600.

Embodiments are also directed to computer program products comprising computer code or instructions stored on any computer-readable medium. Such computer program products include hard disk drives, optical disk drives, memory device packages, portable memory sticks, memory cards, and other types of physical storage hardware.

IV. Additional Example Embodiments

A method for workload optimization in a distributed query processing system is provided herein. The method comprising: receiving a workload comprising a plurality of queries; extracting sets of indexable columns from the plurality of queries, wherein each set corresponds to a respective one of the plurality of queries; generating a set of candidate indexes based on the sets of indexable columns; selecting a predetermined number of candidate indexes from the set of candidate indexes based on a determination of an estimated performance increase provided by the set of candidate indexes to performance of the workload; building the selected candidate indexes to provide built indexes; receiving a query; generating for the query a query plan configured to use at least one of the built indexes; and executing the query plan to generate a final query result.

In an embodiment of the foregoing method, indexable columns comprise columns of a query that correspond to at least one of equality predicates, range predicates, equi-join predicates, group-by lists, or projection lists.

In another embodiment of the foregoing method, said generating a set of candidate indexes comprises, for each query in the workload: creating groups of indexable columns for the respective query based on the set of indexable columns corresponding to the respective query, each group of indexable columns corresponding to a table accessed by the respective query; and generating a set of filter indexes and a set of join indexes for each group of indexable columns; and wherein the set of candidate indexes comprises a union of the set of filter indexes and the set of join indexes generated for each group and for each query.

In one embodiment of the foregoing method, a filter index and a join index each comprise indexed columns and included columns, and wherein: for the filter index, the indexed columns comprise the indexable columns for the respective query that correspond only to equality and range predicates, and the included columns comprise all other indexable columns for the respective query; and for the join index, the indexed columns comprise the indexable columns for the respective query that correspond only to equi-join predicates, and the included columns comprise all other indexable columns for the respective query.

In an embodiment of the foregoing method, the filter index and the join index each further comprise partitioned columns corresponding to the indexable columns for the respective query that correspond to those by which the original source data was partitioned.

In another embodiment of the foregoing method, the determination of the estimated performance increase comprises: determining how frequently each candidate index of the set of candidate indexes corresponds to any query of the workload; and wherein said selecting the predetermined number of candidate indexes from the set of candidate indexes comprises: selecting the predetermined number of candidate indexes that most frequently correspond to any query of the workload.

In one embodiment of the foregoing method, the determination of the estimated performance increase comprises: enumerating subsets of the set of candidate indexes, the subsets including the predetermined number of candidate indexes; for each subset of the set of candidate indexes: determining an estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built; combining the determined estimated costs of the queries of the workload to provide an estimated workload cost for the respective subset; and wherein selecting a predetermined number of candidate indexes from the set of candidate indexes comprises: selecting the candidate indexes that correspond to the subset having the lowest estimated workload cost.

In an embodiment of the foregoing method, said determining the estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built comprises: for each query of the workload: generating a query plan; generating an optimized query plan from the query plan by determining for each candidate index of the respective subset of candidate indexes whether the respective candidate index can be matched by a filter index rule or a join index rule to a table scan in the query plan, and in response to the match being determined, replacing the table scan in the query plan with the respective candidate index; and determining the estimated cost of the respective query by applying a cost model to the optimized query plan.

A distributed query processing workload optimization system configured to receive a workload comprising a plurality of queries is provided herein, the system comprising: one or more processors; and one or more memory devices accessible to the one or more processors, the one or more memory devices storing software components for execution by the one or more processors, the software components including: a query processor, a candidate index generator and an index selector, wherein: the candidate index generator is configured to extract sets of indexable columns from the plurality of queries, wherein each set corresponds to a respective one of the plurality of queries, and generate a set of candidate indexes based on the sets of indexable columns; and the index selector is configured to select a predetermined number of candidate indexes from the set of candidate indexes based on a determination of the estimated performance increase provided in performing the workload by the set of candidate indexes; the query processor configured to: build the selected candidate indexes to provide built indexes; receive a query; generate a query plan for the query, wherein the query plan is optimized to use at least one of the built indexes; and execute the query plan to generate a final query result.

In another embodiment of the foregoing system, indexable columns comprise columns of a query that correspond to at least one of equality predicates, range predicates, equi-join predicates, group-by lists, or projection lists.

In an embodiment of the foregoing system, the candidate index generator is further configured to, for each query in the workload: create groups of indexable columns for the respective query based on the set of indexable columns corresponding to the respective query, each group of indexable columns corresponding to a table accessed by the respective query; and generate a set of filter indexes and a set of join indexes for each group of indexable columns; and wherein the set of candidate indexes comprises a union of the set of filter indexes and the set of join indexes generated for each group and for each query.

In one embodiment of the foregoing system, a filter index and a join index each comprise indexed columns and included columns, and wherein: for the filter index, the indexed columns comprise the indexable columns for the respective query that correspond only to equality and range predicates, and the included columns comprise all other indexable columns for the respective query; and for the join index, the indexed columns comprise the indexable columns for the respective query that correspond only to equi-join predicates, and the included columns comprise all other indexable columns for the respective query.

In another embodiment of the foregoing system, the filter index and the join index each further comprise partitioned columns corresponding to the indexable columns for the respective query that correspond to those by which the original source data was partitioned.

In an embodiment of the foregoing system, the determination of the estimated performance increase comprises: a determination of how frequently each candidate index of the set of candidate indexes corresponds to any query of the workload; and wherein said the index selector is further configured to select the predetermined number of candidate indexes from the set of candidate indexes by selecting the predetermined number of candidate indexes that most frequently correspond to any query of the workload.

In another embodiment of the foregoing system, the index selector is further configured to determine the estimated performance by: enumerating subsets of the set of candidate indexes, the subsets including the predetermined number of candidate indexes; for each subset of the set of candidate indexes: determining an estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built; combining the determined estimated costs of the queries of the workload to provide an estimated workload cost for the respective subset; and wherein the index selector is further configured to select the predetermined number of candidate indexes from the set of candidate indexes by selecting the candidate indexes that correspond to the subset having the lowest estimated workload cost.

In yet another embodiment of the foregoing system, said determining the estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built comprises: for each query of the workload: generating a query plan; generating an optimized query plan from the query plan by determining for each candidate index of the respective subset of candidate indexes whether the respective candidate index can be matched by a filter index rule or a join index rule to a table scan in the query plan, and in response to the match being determined, replacing the table scan in the query plan with the respective candidate index; and determining the estimated cost of the respective query by applying a cost model to the optimized query plan.

A computer program product comprising a computer-readable memory device having computer program logic recorded thereon that when executed by at least one processor of a computing device causes the at least one processor to perform operations for optimizing a workload in a distributed query processing system, the operations comprising: receiving a workload comprising a plurality of queries; extracting sets of indexable columns from the plurality of queries, wherein each set corresponds to a respective one of the plurality of queries; generating a set of candidate indexes based on the sets of indexable columns; selecting a predetermined number of candidate indexes from the set of candidate indexes based on a determination of an estimated performance increase provided by the set of candidate indexes to performance of the workload; building the selected candidate indexes to provide built indexes; receiving a query; generating for the query a query plan configured to use at least one of the built indexes; and executing the query plan to generate a final query result.

In another embodiment of the foregoing computer program product, said operations further comprise generating a set of candidate indexes by, for each query in the workload: creating groups of indexable columns for the respective query based on the set of indexable columns corresponding to the respective query, each group of indexable columns corresponding to a table accessed by the respective query; and generating a set of filter indexes and a set of join indexes for each group of indexable columns; and wherein the set of candidate indexes comprises a union of the set of filter indexes and the set of join indexes generated for each group and for each query.

In another embodiment of the foregoing computer program product, said operations further comprise determining the estimated performance increase by: enumerating subsets of the set of candidate indexes, the subsets including the predetermined number of candidate indexes; for each subset of the set of candidate indexes: determining an estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built; combining the determined estimated costs of the queries of the workload to provide an estimated workload cost for the respective subset; and wherein selecting a predetermined number of candidate indexes from the set of candidate indexes comprises: selecting the candidate indexes that correspond to the subset having the lowest estimated workload cost.

In another embodiment of the foregoing computer program product, said operations further comprise determining the estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built by: for each query of the workload: generating a query plan; generating an optimized query plan from the query plan by determining for each candidate index of the respective subset of candidate indexes whether the respective candidate index can be matched by a filter index rule or a join index rule to a table scan in the query plan, and in response to the match being determined, replacing the table scan in the query plan with the respective candidate index; and determining the estimated cost of the respective query by applying a cost model to the optimized query plan.

V. Conclusion

While various embodiments of the disclosed subject matter have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be understood by those skilled in the relevant art(s) that various changes in form and details may be made therein without departing from the spirit and scope of the embodiments as defined in the appended claims. Accordingly, the breadth and scope of the disclosed subject matter should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents. 

What is claimed is:
 1. A distributed query processing system workload optimization method, comprising: receiving a workload comprising a plurality of queries; extracting sets of indexable columns from the plurality of queries, wherein each set corresponds to a respective one of the plurality of queries; generating a set of candidate indexes based on the sets of indexable columns; selecting a predetermined number of candidate indexes from the set of candidate indexes based on a determination of an estimated performance increase provided by the set of candidate indexes to performance of the workload; building the selected candidate indexes to provide built indexes; receiving a query; generating for the query a query plan configured to use at least one of the built indexes; and executing the query plan to generate a final query result.
 2. The distributed query processing system workload optimization method of claim 1, wherein indexable columns comprise columns of a query that correspond to at least one of equality predicates, range predicates, equi-join predicates, group-by lists, or projection lists.
 3. The distributed query processing system workload optimization method of claim 2, wherein said generating a set of candidate indexes comprises, for each query in the workload: creating groups of indexable columns for the respective query based on the set of indexable columns corresponding to the respective query, each group of indexable columns corresponding to a table accessed by the respective query; and generating a set of filter indexes and a set of join indexes for each group of indexable columns; and wherein the set of candidate indexes comprises a union of the set of filter indexes and the set of join indexes generated for each group and for each query.
 4. The distributed query processing system workload optimization method of claim 3, wherein a filter index and a join index each comprise indexed columns and included columns, and wherein: for the filter index, the indexed columns comprise the indexable columns for the respective query that correspond only to equality and range predicates, and the included columns comprise all other indexable columns for the respective query; and for the join index, the indexed columns comprise the indexable columns for the respective query that correspond only to equi-join predicates, and the included columns comprise all other indexable columns for the respective query.
 5. The distributed query processing system workload optimization method of claim 4, wherein the filter index and the join index each further comprise partitioned columns corresponding to the indexable columns for the respective query that correspond to those by which the original source data was partitioned.
 6. The distributed query processing system workload optimization method of claim 1, wherein the determination of the estimated performance increase comprises: determining how frequently each candidate index of the set of candidate indexes corresponds to any query of the workload; and wherein said selecting the predetermined number of candidate indexes from the set of candidate indexes comprises: selecting the predetermined number of candidate indexes that most frequently correspond to any query of the workload.
 7. The distributed query processing system workload optimization method of claim 1, wherein the determination of the estimated performance increase comprises: enumerating subsets of the set of candidate indexes, the subsets including the predetermined number of candidate indexes; for each subset of the set of candidate indexes: determining an estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built; combining the determined estimated costs of the queries of the workload to provide an estimated workload cost for the respective subset; and wherein selecting a predetermined number of candidate indexes from the set of candidate indexes comprises: selecting the candidate indexes that correspond to the subset having the lowest estimated workload cost.
 8. The distributed query processing system workload optimization method of claim 7, wherein said determining the estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built comprises: for each query of the workload: generating a query plan; generating an optimized query plan from the query plan by determining for each candidate index of the respective subset of candidate indexes whether the respective candidate index can be matched by a filter index rule or a join index rule to a table scan in the query plan, and in response to the match being determined, replacing the table scan in the query plan with the respective candidate index; and determining the estimated cost of the respective query by applying a cost model to the optimized query plan.
 9. A distributed query processing workload optimization system configured to receive a workload comprising a plurality of queries, the system comprising: one or more processors; and one or more memory devices accessible to the one or more processors, the one or more memory devices storing program code for execution by the one or more processors, the program code including: a query processor, a candidate index generator and an index selector, wherein: the candidate index generator is configured to extract sets of indexable columns from the plurality of queries, wherein each set corresponds to a respective one of the plurality of queries, and generate a set of candidate indexes based on the sets of indexable columns; and the index selector is configured to select a predetermined number of candidate indexes from the set of candidate indexes based on a determination of the estimated performance increase provided in performing the workload by the set of candidate indexes; the query processor configured to: build the selected candidate indexes to provide built indexes; receive a query; generate a query plan for the query, wherein the query plan is optimized to use at least one of the built indexes; and execute the query plan to generate a final query result.
 10. The distributed query processing workload optimization system of claim 9, wherein indexable columns comprise columns of a query that correspond to at least one of equality predicates, range predicates, equi-join predicates, group-by lists, or projection lists.
 11. The distributed query processing workload optimization system of claim 10, wherein the candidate index generator is further configured to, for each query in the workload: create groups of indexable columns for the respective query based on the set of indexable columns corresponding to the respective query, each group of indexable columns corresponding to a table accessed by the respective query; and generate a set of filter indexes and a set of join indexes for each group of indexable columns; and wherein the set of candidate indexes comprises a union of the set of filter indexes and the set of join indexes generated for each group and for each query.
 12. The distributed query processing workload optimization system of claim 11, wherein a filter index and a join index each comprise indexed columns and included columns, and wherein: for the filter index, the indexed columns comprise the indexable columns for the respective query that correspond only to equality and range predicates, and the included columns comprise all other indexable columns for the respective query; and for the join index, the indexed columns comprise the indexable columns for the respective query that correspond only to equi-join predicates, and the included columns comprise all other indexable columns for the respective query.
 13. The distributed query processing workload optimization system of claim 12, wherein the filter index and the join index each further comprise partitioned columns corresponding to the indexable columns for the respective query that correspond to those by which the original source data was partitioned.
 14. The distributed query processing workload optimization system of claim 9, wherein the determination of the estimated performance increase comprises: a determination of how frequently each candidate index of the set of candidate indexes corresponds to any query of the workload; and wherein said the index selector is further configured to select the predetermined number of candidate indexes from the set of candidate indexes by selecting the predetermined number of candidate indexes that most frequently correspond to any query of the workload.
 15. The distributed query processing workload optimization system of claim 9, wherein the index selector is further configured to determine the estimated performance by: enumerating subsets of the set of candidate indexes, the subsets including the predetermined number of candidate indexes; for each subset of the set of candidate indexes: determining an estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built; combining the determined estimated costs of the queries of the workload to provide an estimated workload cost for the respective subset; and wherein the index selector is further configured to select the predetermined number of candidate indexes from the set of candidate indexes by selecting the candidate indexes that correspond to the subset having the lowest estimated workload cost.
 16. The distributed query processing workload optimization system of claim 15, wherein said determining the estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built comprises: for each query of the workload: generating a query plan; generating an optimized query plan from the query plan by determining for each candidate index of the respective subset of candidate indexes whether the respective candidate index can be matched by a filter index rule or a join index rule to a table scan in the query plan, and in response to the match being determined, replacing the table scan in the query plan with the respective candidate index; and determining the estimated cost of the respective query by applying a cost model to the optimized query plan.
 17. A computer program product comprising a computer-readable memory device having computer program logic recorded thereon that when executed by at least one processor of a computing device causes the at least one processor to perform operations for optimizing a workload in a distributed query processing system, the operations comprising: receiving a workload comprising a plurality of queries; extracting sets of indexable columns from the plurality of queries, wherein each set corresponds to a respective one of the plurality of queries; generating a set of candidate indexes based on the sets of indexable columns; selecting a predetermined number of candidate indexes from the set of candidate indexes based on a determination of an estimated performance increase provided by the set of candidate indexes to performance of the workload; building the selected candidate indexes to provide built indexes; receiving a query; generating for the query a query plan configured to use at least one of the built indexes; and executing the query plan to generate a final query result.
 18. The computer program product of claim 17, wherein said operations further comprise generating a set of candidate indexes by, for each query in the workload: creating groups of indexable columns for the respective query based on the set of indexable columns corresponding to the respective query, each group of indexable columns corresponding to a table accessed by the respective query; and generating a set of filter indexes and a set of join indexes for each group of indexable columns; and wherein the set of candidate indexes comprises a union of the set of filter indexes and the set of join indexes generated for each group and for each query.
 19. The computer program product of claim 18, wherein said operations further comprise determining the estimated performance increase by: enumerating subsets of the set of candidate indexes, the subsets including the predetermined number of candidate indexes; for each subset of the set of candidate indexes: determining an estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built; combining the determined estimated costs of the queries of the workload to provide an estimated workload cost for the respective subset; and wherein selecting a predetermined number of candidate indexes from the set of candidate indexes comprises: selecting the candidate indexes that correspond to the subset having the lowest estimated workload cost.
 20. The computer program product of claim 19, wherein said operations further comprise determining the estimated cost of each query of the workload under an assumption the candidate indexes of the respective subset of candidate indexes were built by: for each query of the workload: generating a query plan; generating an optimized query plan from the query plan by determining for each candidate index of the respective subset of candidate indexes whether the respective candidate index can be matched by a filter index rule or a join index rule to a table scan in the query plan, and in response to the match being determined, replacing the table scan in the query plan with the respective candidate index; and determining the estimated cost of the respective query by applying a cost model to the optimized query plan. 